The burning of gasoline by conventional spark ignition engines is considered to be a significant contributor to poor air quality, at least in certain geographical areas. As a result, efforts have been undertaken, and are being undertaken, to develop engines which can operate on alternative fuels which burn more cleanly than gasoline. These alternative fuels include natural gas, propane, methanol, ethanol, hydrogen, reformulated gasoline, and biodiesel (i.e., vegetable oil esters). The engines which are being developed to burn these alternative fuels are primarily spark-ignited, light duty engines for use in automobiles.
Electrically powered vehicles, which would not require the direct burning of gasoline or any of the alternative fuels, are also currently being developed. Therefore, presumably, replacing conventionally powered vehicles with electrically powered vehicles would result in a net reduction of the contribution to poor air quality attributable to vehicles. However, while electric motors can replace gasoline engines (typically Otto cycle) and diesel cycle engines (hereinafter, "diesel engines"), electric motors are not yet cost effective, nor are they reliable in heavy duty applications.
Ideally, if diesel engines can be made to burn alternative fuels, the power advantages, the fuel consumption advantages, and the heat rejection advantages of diesel engines over spark ignited engines can be retained. However, except for the case of biodiesel, significant modifications of diesel engines capable of forming fuel oil (hereinafter "diesel fuel") are required to permit alternative fuels to be burned therein.
For example, while diesel engines can be converted to burn natural gas, natural gas engine systems require either a large gas compressor to compress the natural gas, or a cryogenic tank to store liquid natural gas and a high pressure supply pump to pump the liquid natural gas to the combustion cylinders of the engine. Such systems, therefore, involve higher expenses than are involved in conventional diesel engines. These higher expenses associated with such natural gas engines are not easily offset by the lower cost of the natural gas fuel. That is, while the expenses of converting large diesel engines over to natural gas may be offset within a reasonable amount of time by the lower price of natural gas, the expenses of converting smaller engines, such as engines having less than about ten liters of displacement, over to natural gas are not so easily offset by the lower price of natural gas.
Furthermore, while burning biodiesel in a diesel engine does not require significant engine modifications, the cost of biodiesel fuel currently is approximately three times that of conventional diesel fuel.
Propane, however, offers a reasonable alternative. The cost of propane is higher than the cost of natural gas and, in fact, approaches the cost of diesel fuel. Yet, a diesel engine which is converted to burn propane does not require a large gas compressor, or a cryogenic tank and a high pressure supply pump, as does a diesel engine which is converted to burn natural gas. Therefore, although a diesel engine which is converted to burn propane may cost somewhat more than a conventional diesel engine, the cost is less than the cost of converting a diesel engine to burn other alternative fuels and is reasonable considering its less harmful impact on the environment.